1. Field of the Invention
The invention relates to ceramic broadband radomes and, in particular, to a multilayer, prestressed radome, employing alternating layers of silicon nitride and fused silica.
2. Description of the Prior Art
In various types of aircraft and missiles carrying radar equipment, an antenna is mounted in the nose of the craft and is covered with an appropriate aerodynamic surface or radome. The radome must be constructed of material which is strong enough to withstand the aerodynamic forces to which it may be subjected, and yet must be relatively distortion-free and highly transparent to radar energy.
A radome operating at high temperature suitable for aircraft and missiles travelling at supersonic speeds should possess the following properties:
1. high power transmission efficiency over a wide range of radar frequencies;
2. resistance to dust, particle and rain erosion; and
3. resistance to failure due to stresses induced by large thermal differentials.
The electrical materials properties which are most important to the radome designer are the dielectric constant and loss tangent. For acceptable power transmission efficiency, values of less than 10 and 0.01 are required for dielectric constant (.epsilon.) and loss tangent (tan .delta.), respectively. Monolithic or solid wall radomes are designed as electrically thin walls or half-wave walls for maximum power transmission efficiency at the design frequency. Physical design thickness equivalent to an electrical thickness of .lambda./2 decreases with increasing dielectric constant. If the physical thickness becomes too thin to sustain aerodynamic loads, the thickness is then increased by some multiple of two, resulting in a second, third or higher order wall. As the wall order and the angle of incidence increase, however, transmission efficiency decreases. Consequently, the physical thickness of a solid wall radome is trade-off between threshhold loads, incident angle and weight incurred by increasing orders of electrical wall thickness. Transmission efficiency is also influenced by deviation from design thickness. As deviations from the design thickness increase, reduction in power transmission efficiency becomes greater with increasing dielectric constant and frequency. At low frequencies, wall thickness tolerances can be large, whereas at high frequencies, the tolerances are more stringent. Design transmission efficiency can be maintained at a high level with large wall thickness tolerances by utilizing an electrically thin wall design, equivalent to .lambda./10 or less. However, the physical thickness of these walls is very small and is in general too thin to sustain aerodynamic loads.
Ceramic materials with dielectric properties suitable for use as solid wall radomes are generally limited to silicon nitride, alumina, silica, PYROCERAM.RTM. 9606 (trademark of Corning Glass Corp., Corning, NY), cordierite, mullite and beryllia. None of these materials alone as a monolithic wall, with the required dielectric constant of less than 10.0 and loss tangent of less than 0.01, meets the criteria of high transmission efficiency, rain erosion and thermal stress resistance which are required of a radome for protection of antennas operating over a broad frequency range. In general, the broadband transmission efficiency of a solid wall or monolithic radome increases with lower dielectric constant. Slip cast fused silica has the lowest dielectric constant of the above materials (3.36) and exhibits good broadband transmission efficiency over a limited spectrum. Because of its low coefficient of expansion, it is the most resistant of the above materials to failures induced by differential thermal stresses. However, slip cast fused silica offers the poorest rain erosion and particle impact resistance. Alumina, the hardest of the above materials and most resistant to rain erosion and particle impact, exhibits the lowest resistance to failure due to thermal stresses. However, due to its high dielectric constant (9.0 to 10.0 ), alumina is the poorest broadband ceramic radome material.
In addition to the solid wall structures, "A", "B" and "C" sandwiches and multilayer designs can be considered for obtaining high power transmission efficiency over a broad band of frequencies. The "A" sandwich consists of a low dielectric constant core between two high dielectric constant skins. The dielectric constant of the core is usually less than the square root of the skin dielectric constant, with an electrical thickness equivalent to an odd multiple of approximately .lambda./4. The low dielectric constant of the core in a ceramic radome is usually obtained by reducing the density, i.e., increasing porosity. Although "A" sandwiches exhibit efficient power transmission, they are very sensitive to change in frequency and incident angle. Serious degradation of performance occurs above incident angles of 60.degree.. Further, "A" sandwiches fail due to thermal stress differentials above velocities of Mach 3.
The "B" sandwich configuration consists of a dense, thin core of high dielectric constant material, with two thick skins of lower dielectric constant material. The dielectric constant of the core is usually greater than the square of the dielectric constant of the skins; see, e.g., U.S. Pat. No. 3,780,374, which discloses a three layer radome structure comprising a single layer dielectric plate of any desired thickness having a dielectric constant E.sub.r and two dielectric matching layers sandwiching the plate therebetween, each of the matching layers having an average dielectric constant of E.sub.r.sup.1/2 and a thickness which is an odd number multiple of .lambda./4, where .lambda. is the wavelength in the matching layer.
The "B" sandwich generally has a higher power transmission efficiency than the "A" sandwich; but since the low dielectric constant porous material is on the outside of the radome, its use in a high temperature, high performance missile is not practical.
The "C" sandwich configuration consists of two contiguous "A" sandwiches. Sensitivity to frequency, incident angle and polarization is less than with the "A" or "B" sandwiches.
Increasing the number of layers and optimizing the thicknesses result in higher power transmission efficiency over specified broadband spectra and reduce sensitivity to polarization and high incident angles as compared to the "A", "B", or "C" sandwiches; see, e.g., U.S. Pat. No. 3,002,190. Additionally, multilayers offer a higher stiffness-to-weight ratio than conventional monolithic wall radomes. Transmission efficiency in a multilayer system will increase as (1) the dielectric constant of the skin and core decrease, (2) the number of layers increase and (3) the skin thicknesses become smaller. Compliance with the second condition is a function of the rate of gain in transmission efficiency which can be obtaned by increasing the number of layers. Reduction of skin thickness to minimize reflections is a function of the load carrying requirements and fabrication techniques.
Alumina multilayers, optimized for maximum power transmission efficiency over a broadband of frequencies, perform less efficiently than electrically thin walls of silica. Alumina multilayers exhibit excellent particle impact resistance, but perform poorly when subjected to rain erosion at high Mach numbers.
Examples of references disclosing other configurations of radomes include U.S. Pat. Nos. 3,195,138, 3,292,544 and 3,396,396.